Abstract

In preparation of fiber - optic strain measurements on an aircraft wing the performance of an interferometric strain gauge is investigated. A double - polarization Michelson-interferometer is employed for remote sensing of the bending induced surface strain (up to 2500 με) of plates made from Carbon fiber composites (CFRP). Figure 1 shows the arrangement of a surface adhered fiber - optic strain gauge (OSG) with reference (R) and sensing arm (S)) of equal length fixed nearby a conventional resistive gauge (ESG). The fiber jacket is removed in the sensing region. 1 = distance between the supports = 65 mm, L = lenth of sensitive fiber section = 44.4 mm ≥ L_fil,2 = adhesive covered sections with different lengths for different experiments i, 2h = thickness of plate = 1 mm. The double-polarization method is employed for eliminating the ambiguity in fringe counting [1]. Fig. 2 shows a schematic of the experimental setup. A linearly polarized wave from a 1 mW HeNe laser (L) is guided to the interferometer via 5 m of cabled polarization maintaining input fiber which is connected to the input arm of a conventional 3 dB directional coupler via a NTT-FC single mode connector. A polarizing beam splitter (PB) splits the collimated (G) output wave into two orthogonally polarized interference signals (horizontal H, vertical V) with different phase offset (φ_H, Φ_V) which are guided to photodiodes via multimode fibers (MF):

© 1992 Optical Society of America